1. Field of the Invention
The present invention relates to a porous film of polytetrafluoroethylene (hereinafter referred to as "PTFE") and preparation thereof. More particularly, the present invention relates to a porous film of PTFE which has a uniform thin thickness and is suitable for the production of sensors.
2. Description of the Related Art
The PTFE porous film is used as a filter, a water-proofing moisture-permeable clothing material, a sensor film and a cell separator. The PTFE porous film is often fabricated by paste extruding a fine powder of PTFE which is prepared by coagulating and drying emulsion polymerized PTFE powder and stretching an extruded film (first process) (cf. Japanese Patent Publication Nos. 13560/1967 and 18991/1976 and Japanese Patent Kokai Publication Nos. 156067/1979 and 152825/1984). Alternatively, the PTFE porous film can be fabricated by weaving fibers which are prepared by the so-called emulsion spinning of an aqueous dispersion of the emulsion polymerized PTFE particles (second process) or by forming a film from fibrils which are prepared by grinding the PTFE fine powder in a manner analogous to paper making or by sintering such fibrils under pressure (third process) (cf. Japanese Patent No. 21819/1987).
Although the PTFE porous film fabricated by the first process has good strength, the production steps are complicated and productivity is poor. It is difficult to fabricate a film having a thickness of less than 10 .mu.m, since the film is broken during stretching to such a thin thickness. In general, the PTFE porous film fabricated by the first process has a thickness of 30 to 100 .mu.m. In addition, the PTFE porous film fabricated by the first process has a heterogeneous structure which essentially consists of nodes and fibrils, and the node portions have no pores (see FIG. 1 which is a scanning electron microscopic photograph of the PTFE porous film fabricated by the stretching method at the magnification of 10,000x). It is further difficult to fabricate the thin porous film by the second and third processes.
In order to use the PTFE porous film as the film for various sensors with utilizing heat resistance, chemical resistance and compatibility with biological materials of PTFE and to increase sensitivity of the sensor, it is desired to provide a PTFE porous film having a thinner thickness.